1. Field of the Invention
The present invention relates generally to photolithographic optical systems used in semiconductor manufacturing, and more particularly to systems for shaping deformable mirrors within such optical systems.
2. Related Art
Semiconductor devices are usually manufactured using photolithographic techniques. As the demand for ever smaller semiconductor devices has increased, so has the need for ever more precise photolithographic techniques. A crucial element of the photolithographic technology is the optics system.
Today, photolithographic optics systems have been pushed up almost against the physical limitations of the light they seek to manipulate and direct. Tolerances are typically measured in nanometers and this leaves little room for errors. For example, the simple heating or cooling of a mirror in an optics system by only a fraction of a degree, can have severe effects in the performance of the optics system. A substantial heat load is contributed by the absorption of exposure light in a photolithographic lens assembly. Other dynamic environmental factors that can affect system performance are humidity, and barometric pressure. Static errors can also be introduced into these complex systems. Examples of static errors are defects in the mirror itself such as defects introduced by polishing or by minute shape variations. As a result of these potential errors, more inventive and precise ways to compensate for minute environmental and static variations are needed.
In addition to the demand for increasing precision and decreasing size, the semiconductor manufacturing industry has also experienced an ever increasing need for volume. For this reason, photolithographic technology must also be robust enough to stand up to the demands of the manufacturing process. Problems or delays in any part of the process can have serious effects on a companies ability to meet demand.
Therefore, there is a need for precise optical systems, with the ability to dynamically compensate for system aberrations introduced by static errors, environmental variations, or absorption of the exposure light, that can also withstand the rigors of a mass production environment. Our contribution to the art is a precise, robust system to shape a deformable spherical mirror, allowing for correction of various optical aberrations.
A deformable mirror capable of being dynamically adjusted is known in the prior art. See, for example, European patent application EP 0 961 149 A2 entitled xe2x80x9cCatadioptric Objective for Projection with Adaptive Mirror and Projection Exposure Methodxe2x80x9d to Carl Zeiss, which is herein incorporated by reference. Therein disclosed is a deformable mirror with a few independent actuators (two in the preferred embodiment). It is therein described as advantageous to have a small number of actuators. The specific design of actuators themselves is not disclosed.
Another similar system is disclosed in U.S. Pat. No. 5,142,132, entitled xe2x80x9cAdaptive Optic Wafer Stepper Illumination Systemxe2x80x9d issuing Aug. 25, 1992, to MacDonald et al., which is also incorporated herein by reference. Therein disclosed is an optical system containing deformable mirror, in which the deformable mirror is capable of being shaped by a plurality of actuators.
The current invention distinguishes itself from the prior art as being an apparatus and method of carrying out the actual deformation, or shaping, of a deformable mirror. The invention combines the precision necessary in the art, with the robustness required by the manufacturing world.
The present invention comprises a robust system to dynamically adjust the shape of deformable mirrors in photolithographic tools. Optical performance is improved, and the invention is robust enough to sustain multiple individual component failures without decreasing effectiveness.
The invention uses a system of a plurality of pneumatic actuators, placed approximately equidistantly around the perimeter of a deformable mirror, to dynamically correct transmitted wavefront aberrations. The pneumatic actuators make use of a cantilevered arm to gain mechanical advantage. This allows for a greater number of smaller actuators thereby realizing both increased robustness and greater control of the deformable mirror as compared to the prior art.
The invention is intended to correct a plurality of different wavefront distortions. More specifically, astigmatism, three-leaf clover, and four-leaf clover aberrations of any orientation are correctable using the present invention. It is designed to accomplish this correction in such a manner so as to minimize any residual errors that may be imparted to the system. The configuration of the system can accommodate correction of other aberrations as well, and is not limited to the five aberrations mentioned.
Accordingly, it is an object of the present invention to provide a precise and robust system for dynamically adjusting a deformable mirror in an optics system in order to enhance overall optical performance.
It is an advantage of the present invention that the use of a plurality of actuator sets incorporates bending modes with redundant actuation capability enabling continued use with some individual actuator failure.
It is another advantage of the present invention that the pneumatic actuator sets are operated in a self reacting configuration that results in a zero net force and zero net moment being transmitted at the systems boundaries.
It is another advantage of the present invention that the pneumatic actuators use a lever to gain mechanical advantage, which allows a greater number of smaller actuators to provide both greater redundancy and increased control over the deformable mirror. Such actuators are also useful to deform thick mirrors.
It is another advantage of the present invention that the system is easily assembled without inducing any stress or strain into the system.
It is another advantage of the present invention that the pneumatic actuators are easily replaced without inducing any stress or strain into the system, and without affecting the shape of the mirror.
It is yet another advantage of the system that wide manufacturing tolerances are accommodated.
It is yet another advantage of the invention that the configuration of the actuators can correct for arbitrary linear combinations of aberrations from the selected wavefronts.
It is yet another advantage of the invention that the use of non-heat producing actuators eliminates any system thermal disturbances that can be imparted by prior art systems.
These and other objects, advantages and features will become readily apparent in view of the following detailed description of the invention.